Especially, but not exclusively in the field of piping of hazardous gases, it is customary to couple adjacent lengths of tubing by means of a compression-type union. Such tubing is generally made of a strong corrosion resistant metal, and each segment terminates at a hard peripheral sealing surface.
These beads are opposed to one another across a seal washer. Compressive coupling means, comprising a pair of threaded-together nuts, force the segments toward the washer so as to indent the seals into the washer to make a fluid seal. The nuts bear against respective shoulders on the segments, and the segments project through central passages in the nuts.
The objective of these unions is to compress the two beads against the washer as a stack. A union of this type has been in widespread usage for many years. In one of its most commonly-encountered applications, it is used to couple segments of metal tubing used in chemical process equipment--for example in chip manufacturing installations, where it is referred to as a VCR (vacuum compression rad) union. For purposes of making a highly reliable fluid tight seal between the tubing segments, it is fully adequate and is widely used.
The VCR fitting requires that the two beads be tightened down against the washer with a combined rotary and axial motion derived from the tightening of the threads which join the two nuts. If the ends of the tubing segments removed from the union are free to rotate, there is no problem. However, this is usually not the situation. Instead, these tubing segments are customarily already rigidly connected to some next element. As a consequence, rotation of the tubing segments caused by tightening of the union results in deformation of the segments as a consequence of what had also become a twisting force. What is intended to be a neat assembly turns into a group of randomly distorted tubing segments. This presents a less-than-professional appearance for what is often a high-cost piece of capital equipment.
Worse still is the fact that this deformation stores restorative energy tending to loosen the union. Considerable care must be taken to assure that the union is tight enough to prevent this. Even so, events such as earthquakes cause concern that such unions might become loose.
Attempts are made, of course, to attempt to avoid these consequences. Careful installation can at least reduce it, and thrust washers have been suggested as a means to reduce the rotational forces exerted on the beads. Such arrangements add complexity and expense to a unit which ought to be kept as elegantly simple as possible.
It is an object of this invention to provide a simple union in which the bead seals, while being tightened, are exposed only to axial compressive sources, and do not rotate relative to the washer. The objections to the conventional union are thereby entirely overcome and this is done with a simple restraint.